In recent years, a cell-phone and a mobile information terminal both carrying an image pickup apparatus are growing popular, with a trend to attain a highly efficient and small-sized image pickup apparatus employing a solid image pickup element such as a CCD (Charge Coupled Device) type image sensor or a CMOS (Complementary Metal Oxide Semiconductor) type image sensor. Further, demands for the image pickup lens mounted on the image pickup apparatus to be smaller in size and to be higher in efficiency are becoming stronger. As an image pickup lens used for the aforesaid uses, there is suggested a four-element image pickup lens because of the reason that acquisition of higher efficiency is more probable for the four-element image pickup lens than for a two-element or three-element image pickup lens.
As this four-element image pickup lens, there is disclosed the so-called inverted Ernostar type image pickup lens which includes the first lens having positive refractive power, the second lens having negative refractive power, the third lens having positive refractive power and the fourth lens having positive refractive power in this order from the photographic object side, to aim high efficiency (for example, see Unexamined Japanese Patent Application Publication No. 2004-341013).
Further, there is disclosed the so-called a telephoto type image pickup lens which includes the first lens having positive refractive power, the second lens having negative refractive power, the third lens having positive refractive power and the fourth lens having negative refractive power in this order from the photographic object side, to aim downsizing of a total length of the image pickup lens, where the total length means a distance on the optical axis from the lens surface closest to the object in the total system of the image pickup lens to the focal point on the image side (For example, see Unexamined Japanese Patent Application Publication Nos. 2002-365529, 2002-365530, 2002-365531, and 2005-292559).
However, since the image pickup lens described in the Unexamined Japanese Patent Application Publication (JP-A) No. 2004-341013 has a positive fourth lens due to the inverted Ernostar type, the image pickup lens provides a principal point position of the optical system closer to the image side and provides a longer back focus, compared with an image pickup lens of telephoto type which provides a negative fourth lens. Therefore, such type of image pickup lens as described in JP-A 2004-341013 is disadvantageous for downsizing. In addition, there is provided only one lens having negative refractive power among four lenses in the image pickup lens. Thus, it provides a difficulty in correction of Petzval's sum, and in securing excellent performance on the periphery portion of the image area.
The image pickup lens described in each of JP-A 2002-365529, 2002-365530, and 2002-365531 has a narrower imaging field angle and its aberrations is insufficiently corrected. If the total length of the image pickup lens is further shortened, it becomes difficult to be used with an image pickup element with a large number of pixels due to deterioration of its performance.
In addition, the image pickup lens described in JP-A No. 2005-292559 has a long back focus in spite of its telephoto type, because the negative refractive power of the fourth lens is relatively weak, and cannot attain sufficient downsizing. Further, aberration correction is also insufficient in the view of coping with an image pickup element with a large amount of pixels.